The invention relates to a side-looking SAR (synthetic aperture radar) system.
Limitations of a Conventional SAR System
For conventional SAR systems, the coverage in across track direction and geometric resolution in along track direction are contradicting system parameters.
In a conventional monostatic SAR system, the same real aperture of length L and height H is used for transmit and receive. In order to sample the radar echoes of the wanted target area unambiguously, it is shown in J. C. Curlander and R. N. McDonough, Synthetic Aperture Radar Systems and Signal Processing, New York: Wiley, 1991, p. 21 ff. that a minimum antenna aperture A is required.                     A        =                              L            ·            H                    >                                    4              ·                              v                s                            ·              λ              ·                              R                s                            ·                              tan                ⁡                                  (                  φ                  )                                                      c                                              Equ        .                                   ⁢        1            
In Equ. 1, υs is the speed of the SAR platform, λ is the wavelength at center frequency, Rs is the slant range to the target, φ is the incidence angle and c is the speed of light. Even though Equ. 1 is based on a number of approximations, it clearly shows the principle limitations of a conventional SAR system. The two top level system parameters swath width Wsw  and the azimuth resolution δaz are contradicting and can not be improved at the same time. In order to illuminate a wider swath width, the antenna height H must be decreased. A better azimuth resolution in stripmap mode requires a shorter length L of the antenna (δaz=L/2).
For the case of an airborne SAR, this constraint is not so important because the platform speed υs and the slant range Rs are orders of magnitude smaller, than in the spaceborne case. The minimum antenna size is a very important consideration in the spaceborne case. Conventional SAR systems use special modes of operation to overcome these constrains. They are called the Spotlight and the ScanSAR mode described in A. Currie, M. A. Brown, Wide-swath SAR, IEE Proceedings-F, Vol. 139, No. 2, April 1992.
The Spotlight mode allows to improve the azimuth resolution by pointing the antenna beam to the spot for a longer aperture. The disadvantage is that by doing so, only single high resolution spots can be imaged, but no continuous coverage is possible.
The ScanSAR mode uses a highly agile antenna beam in order to switch rapidly between a number of N subswath. This results in an improved swath width but at the cost of a N+1 times reduced azimuth resolution.
DE 34 30 749 A1 describes a method of swath widening and data reduction in a SAR system. The method utilizes the fact that the Doppler history for targets from different distance ranges has slight differences. The echos of targets from different distance ranges are received in one single receive channel and transmitted to the ground as just one echo. There, the echos of different distance ranges can be separated due to their individual Doppler histories.
The system described in R. Kwok, W. T. K. Johnson, Block Adaptive Quantization of Magellan SAR Data, 1989, IEEE Trans. Geoscience & Remote Sensing, Vol. 27, No. 4, pp. 375-383 has a special mode for improved along track resolution During receive, the aperture is divided in azimuth into two sub-apertures and the signal of each sub-aperture is separately recorded and transmitted to the ground for SAR processing. The same division in azimuth can be used for the detection of moving targets.
The principle of moving target detection is described in detail in P. Meisl, A. Thompson, A. Luscombe, RADARSAT-2 Mission: Overview and Develop-ment Status, Proceedings of EUSAR 2000[1] J. H. G. Ender, Detection and Estimation of Moving Target Signals by Multi-Channel SAR, AEÜ Int. J. Electron Commun. 50(1996) No. 2, 150-156. It requires multiple receive channels and multiple receive antennas or sub-apertures separated in along track direction. Special signal processing algorithms then allow detection of moving targets within the SAR image.
A further technique, using two receive apertures and receive channels, is the SAR interferometry described in Fuk K. Li, R. M. Goldstein, Studies of Multibaseline Spaceborne Interferometric Synthetic Aperture Radars, IEEE Transactions on Geoscience and Remote Sensing, Vol. 28, No. 1, January 1990. There the two receive apertures have to be separated in elevation or cross track direction. The separation required for interferometry is in the order of several tens or even hundreds of meters. Here again, the two signals have to be separately recorded and are combined only after the SAR image processing.
The object of the present invention is to overcome the described constraints of a conventional SAR system. The new SAR system should allow combining a high azimuth resolution with an improved swath width and a continuous lossless coverage in stripmap mode.
According to the invention, the SAR system is a bistatic radar, where the receive antenna is built up from multiple receive sub-apertures in azimuth as well as in elevation direction. A coherent data processing to reduce the data volume is performed on board with the signals from the sub-apertures.
The SAR system for spaceborne application is capable of combining a very high geometric resolution with a very large coverage area. Such a SAR system is, e.g., well suited for large area surveillance and high resolution mapping applications. In particular, the SAR system according to the invention allows combining a very high azimuth resolution with an improved swath width.
A higher coverage in across track as well as a higher geometric resolution in along track direction require both an increased average transmit power in a conventional SAR system. The SAR system according to the invention allows the reduction of the required average transmit power by the use of a receive antenna with higher antenna gain and the optimized design of the separated transmit and receive antennas.